This invention relates to an improved process and apparatus for forming articles from molten materials including an innovative mold valve gating apparatus and a mold valve gating method useful for injection molding articles of various shapes.
Various valving arrangements have been proposed in the prior art for regulating the flow of heated plastic material from a source of such material to a mold cavity space. In most instances, regulation of the flow of the melt between a hot runner or cold runner nozzle, through a mold gate and into the mold cavity space, is satisfactorily achieved using a valve stem located in the melt channel of the nozzle. The valve stem is actuated by a motive means, typically located in the mold back plate, to open and close the access of the melt to the mold cavity space. This approach has several drawbacks when it has to be applied to a multicavity mold used to form demanding articles, such as blowable preforms, molded using single or multiple materials. One drawback is where the system requires the use of multiple valve stems. In such an arrangement, the individual actuation of the stem creates problems when it comes to ensuring that all the valve stems are opened and closed simultaneously. Second, the valve stem has the tendency to split the flow of molten material, thus creating the so called unacceptable knit lines. Additionally, the actuation of the stems becomes very problematic for multimaterial injection nozzles when at least two materials are injected in the same cavity space. One known approach intended to solve the first two problems is to use a lateral valve gating arrangement comprising a gate orifice. A gate orifice is shown in U.S. Pat. No. 4,108,956 to Lee.
The approach shown in the Lee patent is not very effective. While it solves the first and second aforementioned drawbacks, it does not simplify the mold design and operation. In fact, the approach complicates both mold design and operation. The valving arrangement shown in Lee involves the use of a reciprocally movable slide member having at least one opening. The slide member is interposed between a source of heated plastic material such as a hot runner outlet or injection nozzle and a mold gate. As can be seen in both of these patents, the movable valve carries with it a hot slug of the plastic material being molded as the valve moves from a valve open to a valve closed position. During the cooling step, the slug is solidified. The presence of such a slug in the valve opening requires the apparatus to include some additional and specialized mechanism for removing it from the valve opening. Should the plug not be ejected from the valve opening, it will be brought back to the gate area and injected into the cavity space with the next shot. In many applications, this is not acceptable as it lowers the cosmetic and strength characteristics of the molded article. This aspect becomes even more critical in multimaterial molding where mixing two or more different materials must be avoided.
In the Lee Patent, a movable mechanical ejection unit actuated by pneumatic means (that is as complicated as the similar means used to move a classical valve stem) is positioned on the molding machine to remove the cold slug from the valve opening. Other means, not shown by Lee, must be further used to remove the slugs from the mold. The use of such means becomes very difficult in systems utilizing a multicavity mold, such as the molds having more than sixteen cavities manufactured by the assignee of the instant application. For example, the molds would become very big and heavy if they had to accommodate movable mechanical ejection devices. Further, additional detection means would be needed to make sure that the slug has been indeed ejected from each opening.
A machine, such as that shown in the Lee Patent, is further economically disadvantageous in that the slug is wasted material that could otherwise be used to fabricate molded articles. In addition to the slug removal and waste problems, these systems must face the potential problem of having molten plastic material flow between the surfaces of the valve and the adjacent mold plate and hot runner housing. Should molten plastic material flow between these surfaces and reside therein, operation of the valve and the machine itself could be interrupted if the plastic material solidifies.
The latter valve gating arrangement of Lee has been previously used to control flow of the melt from the machine injection nozzle to a mold as shown in U.S. Pat. No. 3,632,729 to Bielfeldt. Certainly, the valving arrangement shown in the Lee patent was not conceived to be used or adapted for valve gating a multimaterial injection nozzle such as that disclosed in U.S. Pat. Nos. 4,863,665; 5,200,207; 5,143,733; 5,112,212; 4,863,369; 4,808,101; 4,775,308; 4,717,324; 4,701,292; and 4,657,496, all assigned to the assignee of the instant application and all which are hereby incorporated by reference herein. Lee""s approach would not be useful in multimaterial molding because it would generate too much waste made of various materials. Further, the need to handle more than one type of slug would become prohibitive both technically and price-wise.
It is also known in the prior art to remove a crystallized gate vestige from a parison (preform) by using reciprocally movable cutting blades. Such a system is shown in U.S. Pat. No. 4,380,423 to Aoki. While the Aoki patent addresses the problem of removing the sprue from the already molded article, it does not address the critical problem of how to open and close off the flow of molten material from a hot runner system to a mold gate without forming the crystallized gate vestige that needs to be cut in a post molding operation. Similar to Lee""s slug, the crystallized gate vestige shown by Aoki represents a significant waste of valuable resin, incurring the problems previously mentioned.
There remains a need for a simpler and more efficient mold valve gating arrangement for a multimaterial injection nozzle. There also remains a need for a mold valve gating arrangement for injecting multiple or single materials wherein the flow of molten material from a hot or cold runner system or injection nozzle to a mold gate can be interrupted without incurring waste of the material being molded. There further is a need for a mold valve gating arrangement which does not require the inclusion of a movable and/or a mechanical ejection unit for removing plastic material being molded from the valve.
A principal object of the present invention is to provide a simpler and more effective valve gating apparatus and method for operating a hot or cold runner injection nozzle that is easier to manufacture, operate and service to injection mold improved articles.
Another object of the present invention is to provide a valve gating apparatus and method wherein the valve gating means is located outside of the injection nozzle.
Another object of the present invention is to provide a valve gating apparatus and method wherein a thin, movable blade including a gate orifice of minimal volume having a fixed aperture is used in lieu of a valve gate stem.
Another principal object of the present invention is to provide a valve gating apparatus and method wherein substantially no material is diverted from the stream of molten material by the movable gate orifice during the injection process.
Another principal object of the present invention is to provide a simpler and more effective valve gating method and apparatus for single material and multimaterial molds having the aforementioned attributes.
Another object of the present invention is to provide an apparatus and method of molding plastic articles made of single or multiple materials with substantially no gate vestige.
Another object of the present invention is to provide an apparatus and a method of molding plastic articles made of single or multiple materials with no crystallinity.
Another object of the present invention is to provide an apparatus and a method of molding plastic articles with no knit lines.
The present invention is carried on in a mold having one or more injection nozzles which guide at least one stream of molten material desirably having a tubular flow pattern towards one or more cavity spaces. More particularly this invention discloses thin movable valve gating arrangements located outside the single material or multimaterial injection nozzle and a method of valve gating a mold where substantially no material is diverted from the molten stream during the transfer of the molten material from the single material or multimaterial injection nozzle to the mold cavity space. Accordingly, substantially no residual material is generated by the movement of the valve gating means. Numerous advantages are obtained in accordance with the present invention and by having substantially no residual slug of material in the mold. For example, the mold is much simpler to design and manufacture having less mechanical or movable parts, easier to build, operate, and service and no additional equipment is needed to eliminate or cut the sprue after molding the article. In addition, almost no molten material is wasted and thus the mold can be operated in a smaller, clean room environment. Still further, the molded article has a higher mechanical resistance and higher aesthetic value.
In accordance with the present invention, an apparatus for forming molded articles is provided which broadly comprises: an injection mold having one or more mold cavity spaces and mold gates communicating with the mold cavity spaces; one or more injection nozzles for feeding molten plastic material to the injection mold cavity space(s) having at least one nozzle outlet therein in line with the mold gates; movable valve gating means, such as a blade, is positioned between the mold gate and the at least one nozzle outlet and has an orifice therein, with the orifice having a minimum volume; means for moving the valve gating means between a first position wherein the orifice communicates with the nozzle outlet and with the mold gate and permits the flow of molten plastic from the at least one outlet to the mold gate, and a second position blocking flow from the at least one nozzle outlet to the mold gate, and the valve gating means being sufficiently thin that during movement of the valve gating means between the first and second positions substantially no plastic material is carried or diverted by the valve gating means.
In accordance with the present invention, a method for forming molded articles is provided which comprises the steps of: feeding at least one molten material from an injection nozzle via at least one nozzle outlet to a mold cavity space of an injection mold through a mold gate substantially in line with the at least one nozzle outlet; and substantially preventing waste of the molten plastic material during molding by positioning a thin, movable valve gating means, with an orifice therein, between the mold gate and the at least one nozzle outlet and moving the valve gating means between a first position wherein the orifice communicates with the at least one nozzle outlet and mold gate and permits the flow of molten plastic from the at least one nozzle outlet to said mold gate, and a second position blocking flow from the at least one nozzle outlet to the mold gate, without substantially any of the molten plastic material being transferred by the thin valve gating means during movement of the valve gating means between the first and second positions.
The apparatus and the method of the present invention have been found to have particular utility in injection molding systems which include multimaterial injection nozzles for injecting two or more materials into a mold cavity space. The valve gating means of the present invention may be designed to permit sequential ejection of the materials into the mold cavity space or simultaneous ejection of the materials into the mold cavity space, all while being thin enough that the valve gating means carries substantially none of the material being molded as it moves between various positions and generates no residual material which must be disposed of. When injecting multiple materials, valve gating means in accordance with the present invention may have more than one gate orifice.
In accordance with a further aspect of the present invention, the valve gating means is desirably positioned very close relative to the mold cavity space so that the height of the residual sprue or gate vestige is minimum. In accordance with another aspect, the gating means is desirably maintained at a temperature so that substantially no crystallinity is generated in the area of the molded article adjacent to the mold orifice or gate. When injecting materials such as PET, no knit lines are formed in the molded article. The valve gating means could be a single blade, which is preferably flexible, plural blades moved in opposite directions, a disk member, or a cup-shaped member. Various actuation means for moving the valve gating means, individually or all together, may be provided on one side or on both sides of it. In addition, since no valve stem resides inside the melt channel of injection nozzle in the present invention, the molded article does not have knit lines and a much faster injection cycle can be achieved.
Further features of and advantages of the present invention will appear hereinbelow.